Gollum finds the One Ring in the Misty Mountains before Bilbo Baggins.
Gollum originally belonged to a river-dwelling people and was called Sméagol. On his birthday, his cousin Déagol discovered the Ring in the River Anduin, and Sméagol killed him to take it. The Ring’s influence gradually twisted Sméagol into the isolated creature known as Gollum.
Gollum lived for centuries beneath the Misty Mountains, where Bilbo encountered him during the events of The Hobbit. Bilbo won the Ring from Gollum in a riddle game and escaped from the tunnels, while Gollum remained unaware at first that the Ring had been taken.
This differs from the film version’s emphasis on the Ring falling into Gollum’s possession after Déagol finds it, but the essential sequence is the same: Gollum possessed the Ring long before Bilbo did. Bilbo’s discovery of it sets the central conflict of The Lord of the Rings in motion.